For safety and to comply with state and federal regulations, truck drivers must regularly measure, adjust and set air brakes. In particular, slack should be checked frequently, definitely before a trip, to determine if pushrod travel is within safe-working limits. Slack adjustment entails adjusting the brakes to keep air chamber pushrod travel and brake shoe travel within tolerance to ensure the brakes are effective. Pushrod travel means the distance the pushrod extends from the air brake chamber when the brakes are applied. Brake shoes should be adjusted to move a predetermined distance before the brake pads impinge against the interior surface of a brake drum. This distance denotes the slack in the pivotal movement of the slack adjuster. Failure to maintain slack within acceptable limits may result in braking failure, such as an inability to stop, increased braking distance, or uneven braking forces causing the vehicle to yaw or roll. Only when all brakes are properly adjusted will the system be able to absorb the required amount of energy to bring the loaded vehicle to a safe stop under extreme conditions.
With reference to FIGS. 1 and 9, a standard air brake assembly 945 used on each wheel includes an air chamber 910 with a reciprocating piston 905. A proximal end of a pushrod 915 is connected to the piston. The pushrod 915 extends outwardly from the piston 905. Pressure from compressed air in the air chamber 910 drives the piston 905. Compressed air flow 900 to the chamber 910 is controlled by a valve operated by the vehicle driver. Upon applying the brakes, the compressed air 900 causes the piston 905 and pushrod 915 to travel. A slack adjuster 100 operably coupled to the pushrod 915 transmits reciprocating motion of the pushrod into rotational motion of a cam configured to controllably urge brake shoes towards a brake drum. The slack adjuster 100 includes an arm and a base. The arm 115 of the slack adjuster 100 is connected by a clevis to a distal end of the pushrod 915. A base 135 of the slack adjuster 100 is pivotally mounted to a camshaft. Reciprocating motion of the pushrod 915 causes pivotal movement of the slack adjuster, which transmits torque through the camshaft. A cam 920, such as an S-shaped cam, is fixedly secured to the opposite end of the camshaft. Depending upon direction, rotation of the cam 920 either transmits or relieves a force through cam-follower rollers 950 that are mounted to the brake shoes 935. The brake shoes pivot about pivot pins 940. When transmitting the force, the cam urges 920 the brake shoes 935 apart towards the drum so that the brake pads 925 mounted on the shoes 935 contact the brake drum 930, thereby bringing the vehicle to a stop. When relieving the force, the brake shoes 935 retreat from the drum 930 so that the brake pads 925 mounted on the shoes 935 cease contact with the brake drum 930, thereby allowing the vehicle to move without resistance.
As the brake drum 930 and pads 925 of the brake shoes 935 wear through use, the slack adjuster 100 has to pivot a farther to apply the brakes. Thus, the range of slack adjuster 100 movement and pushrod 915 travel increase. This range of motion must be delimited so that it does not exceed a certain maximum amount. For this reason, the slack adjuster 100 includes a mechanical means for adjusting and resetting the range of its angular motion.
With reference to FIG. 1, the slack adjuster 100 is not directly mounted on the camshaft but, instead, it is rotatably mounted on a gear 120 that has internal splines that interconnect with an internal worm gear (the “worm”). The slack adjuster can rotate on the gear 120 within a limited range, thereby increasing or decreasing slack by changing the angle of the slack adjuster relative to the cam shaft. The worm rotates on an axis that is perpendicular to the axis of the camshaft. The worm has a polygonal head 130 accessible from outside the slack adjuster 100 to allow the worm to be rotated on its axis by a wrench or socket. Excess slack is taken up by rotating the head 130 to adjust the angle of the slack adjuster 100 to the appropriate position thereby causing the cam 920 to commence applying force to the cam follower rollers 950 concomitant with the movement of the pushrod 915 from the air chamber 910—the brake-on position—a short distance that is generally between ½ inches and ¾ inches from the position of the piston rod when in the brake-off position.
Many slack adjusters have a spring-loaded pawl assembly that meshes with internal teeth, locking the worm contained within the body of the slack adjuster in place. To allow rotation of the worm for adjustment, the spring-loaded pawl must be disengaged. Disengagement entails urging a pawl cap 140 away from the body 135 of slack adjuster 100 about ¼ to 1/32 of an inch. The terms pawl cap and cap are used herein interchangeably and denote a head structure of the pawl assembly which may be urged away from the slack adjuster 100 to cause movement of the pawl shaft 142, thereby disengaging the pawl assembly. Typically, the cap 140 is pried away from the body of the slack adjuster and held, using a screwdriver. Unfortunately, however, locating the cap 140 with a screw driver can be difficult. Additionally, an ordinary screw driver will fall from place if it is let go while prying the cap 140. Thus, the driver must dedicate one hand to holding the tool 200 and maintaining prying action.
What is needed is a reliable and easy-to-use tool for disengaging the pawl of a slack adjuster while an adjustment is made. The invention is directed to overcoming one or more of the problems and solving one or more of the needs as set forth above.